Special machines, known as automatic coil-making machines, are known for the manufacture of wire coils, which are used as coil springs and particularly as electric resistance heating coils, and are made from wire in various thicknesses, qualities, with various numbers of convolutions and in various diameters. In these machines, the wire is fed by a feeder to a winding station, in which the wire is shaped to form a continuous helix. The winding station is succeeded by cutting means, which cut wire coils having a predetermined length from the continuous helix. The winding station is preceded by a sensing wheel, which serves to control the length of the wire coils. The wire runs over the sensing wheel to rotate the same in dependence on the length of the wire which is to be processed so that a predetermined angular movement of the sensing wheel corresponds to a certain length of wire. The angular movement of the sensing wheel may be utilized to control the cutting means and the latter may be adjusted to cut off a coil which has been formed from a predetermined length of wire. In practice, the overall length of wire from the point where the wire leaves the sensing wheel to the point where the cutting tool engages the helix is constant so that the points at which the sensing wheel contacts the wire being fed and the point at which the helix is acted upon by the cutting means may be freely selected.
Difficulties are encountered in a known machine of this kind because the continuous wire helix cannot be cut through with the accuracy which is required. One of these difficulties resides in that the preselected wire length which is to be measured to initiate the cutting of the helix cannot be exactly controlled. In the known machine, the control means for the cutting means comprise a camwheel, which is driven by the sensing wheel through the intermediary of an adjustable friction wheel transmission and which actuates two microswitches. One of these two microswitches delivers a control pulse which causes the cutting knife of the cutting means to descend to such an extent that is slides in between two successive convolutions of the helix. The closing of the second microswitch results in the renewed energization of a solenoid which controls the knife or other cutting tool of the cutting means so that the knife is operated to chop through the helical wire.
The friction wheel transmission has a certain slip so that even without a change of the setting of this transmission, different lengths of wire, corresponding to different lengths of the helix, may pass through during successive measurements until the operation is due to the fact that the microswitches are actuated by a camwheel acting on a contact spring and that this actuation cannot be accurately performed and is not always accomplished when the contact spring engages the same point of the camwheel. These circumstances result in considerable length tolerances of the coils. As a result, springs may be unusable for the intended purpose and the resistance of resistance heating coils cannot be exactly controlled, so that there is a considerable percentage of rejects having an excessively low or high mat output.
In the machine which has been described, the continuous helix which leaves the winding station rotates at a speed which corresponds to the winding speed and which is often relatively high, in a range of, e.g., 500-5000 r.p.m. The number of cycles to be performed by the cutting means per unit of time depends on the winding speed and on the selected length of each wire coil. In an operation involving relatively thin wires, short wire coils, and high winding speeds, the time between two successive cutting steps may be less than 1 second. During this short interval of time, the knife must be lowered into the helix, the cut must be performed, and the knife must be returned to its initial position. To avoid an upsetting of the helix, a knife may be used which is moved also in the direction in which the helix advances while the knife is moved between the position in which it initially engages the helix and the position in which it cuts through the helix, which is moved, e.g., along an arc of a circle. As has been described hereinbefore, the cutting time cannot be exactly controlled in dependence on length in the known machines, as has been described hereinbefore. In addition to the disadvantages mentioned above, this inaccuracy has the result in practice that in numerous severed wire coils the wire convolution at which they have been severed has been bent over the opening of the coil. A neatly cut off end of the coil is essential for the further processing of the coil, e.g., in the manufacture of tubular heaters. In the manufacture of tubular heaters, a terminal bolt is inserted into the opening of the coil and is then welded to the coil. A coil having a distorted end portion, as described above, cannot be used and the bent wire end protruding over the opening of the coil must be pinched off by hand before the coil can be processed further. In practice, a plurality of auxiliary workers must work in association with each automatic winding apparatus for the single purpose of inspecting the coils for neatly cut off ends and of pinching off any bent wire portions at such ends. As has been mentioned, the main cause for the bending of the wire portion at the end of the coil is the inaccurate length control, which has the result that during its initial descent, which precedes the actual cut, the knife does not enter between two wire convolutions but bends the wire. If the knife descends too soon, the rotating helix will act on the knife for a relatively long time and may thus cause the knife to vibrate. If the knife descends too late, it will not enter the space between successive convolutions.
For this reason it is an object of the invention to eliminate these disadvantages and to provide a machine which enables an exact setting of selected coil lengths and a manufacture of coils in lengths which deviate only very slightly from the set value.